A zero crossing comparator is used in switching mode power supply (SMPS) to detect when current from an inductor in the power supply reaches zero and at that moment block, or turn off the inductor current. The precision, by which the zero crossing comparator operates, affects the efficiency of the power supply. If the current from the inductor is turned off late (after the zero crossing) the current in the inductor goes negative (overshoot) and a node voltage of the power supply goes above the input voltage turning on a parasitic diode and pushing the extra charge into Vin. If the current from the inductor is turned off early, current from the inductor is connected by a parasitic diode connected to ground, which increases losses since the voltage across the parasitic diode is higher than in the normal on state.
FIG. 1 is a simplified schematic of a typical buck type switching mode power supply (SMPS). The PMOS transistor P and the NMOS transistor N direct the current to and from the inductor L. The node voltage VLX is shown in FIG. 2, wherein the inductor current IL is turned off at the zero current crossing in condition A, late in condition B and early in condition C. Given the variations in the devices that make up the components in FIG. 1 and the very small signal input into the zero compare circuit ZC, the resulting variations in inductor current IL and the subsequent output voltage Vout across the load Rload will appear much like that in FIG. 2 for a typical production of the circuit. Improvement in the circuit design is needed if a more consistent circuit response is required.
US 2004/0027101 A1 (Vinciarelli) is directed to an apparatus comprising a buck-boost DC to DC switching power conversion, wherein a first switching device is interposed between a source and a first terminal of an inductor and a second switching device is interposed between the second terminal of the inductor and the load. U.S. Pat. No. 8,274,266 B2 (Englehardt et al.) is directed to a power supply system that comprises an inductor device and a plurality of switching devices that allow the power supply to operate in a boost mode. U.S. Pat. No. 8,143,874 B2 (Templeton) is directed to a switch mode power supply where an integrated circuit provides ease of integration with switch mode power supply (SMPS) designs. U.S. Pat. No. 8,115,459 B2 (Prodic et al.) is directed to a digitally controlled DC-DC converter with a power stage with at least one switch and an output capacitor. In U.S. Pat. No. 7,893,674 B2 (Mok et al.) a switch mode power supply (SMPS) is directed to a transient recovery circuit to stabilize the circuitry when a transition to a new output is performed. U.S. Pat. No. 7,554,310 B2 (Chapuis et al.) is directed to a switch mode voltage regulator comprising dual digital control loops. U.S. Pat. No. 7,447,049 B2 (Garner et al.) is directed to an SMPS controller using primary side sensing to detect a point of zero magnetic flux. U.S. Pat. No. 6,879,136 B1 (Erisman et al.) is directed to an inductor current emulation circuit for a switch mode power supply configured such that the inductor current goes to zero at least once during a cycle.